This invention relates to magnetic field sensing apparatus and, more particularly, to such apparatus which utilizes a magnetic sensor formed of magnetoresistive elements that is readily adapted to produce an output signal which is linearly related to the displacement of the sensor relative to a source of a magnetic signal field, whereby the sensor can be used in a servo control system, a position control system, and the like.
Many control systems, and particularly analog control systems such as servo control apparatus, employ magnetic sensing devices for use in sensing displacement of the controlled element, position of a controlled device, and the like. Such magnetic sensor devices generally have been used in place of conventional potentiometers for the purpose of producing and feeding back position-representing signals to, for example, a motor control system. In general, such magnetic sensing devices operate with relatively low power requirements, exhibit favorable longevity and are relatively simple to manufacture and assemble. Hence, such magnetic sensing devices find ready application in machine control systems, such as in driving a machine tool to a desired location, maintaining a driven element in a predetermined position in place of, for example, a mechanical brake, and the like.
One type of magnetic sensor that is used in control systems of the aforementioned type, and particularly, magnetic sensors which are used to control the rotational position of a rotary member such as a spindle, generally are used in conjunction with a magnetic element, such as a magnet or a magnetic material, that is secured to the periphery of the rotary member. The magnetic sensor then comprises a suitable pick-up device which is fixedly disposed adjacent the rotary path of this magnetic element. A typical magnetic pick-up device is adapted to sense a change in the magnetic flux coupled thereto due to the rotary presence of the magnetic element. The pick-up device may comprise a conventional magnetic head of the type which is provided with a magnetic core having a detecting coil wound thereabout so as to form a magnetic circuit, this magnetic circuit having a magnetic gap that is sensitive to changes in flux. Another type of magnetic pick-up device which has been used is a ferromagnetic magnetoresistive element which is provided with a ferromagnetic metal material whose resistivity varies as a function of the magnetic field applied thereto. One example of a suitable magnetoresistive element which employs this magnetoresistive effect is described in U.S. Pat. No. 3,928,836. Still another type of magnetic pick-up device which has been used is the semiconductor element which also exhibits a magnetoresistive effect, such as a Hall effect device.
Desirably, the magnetic pick-up device which is to be used in a control system, such as a servo control system, should be relatively sensitive to the displacement therefrom of the magnetic element. That is, the pick-up device should produce an output voltage which changes by relatively large increments in response to relatively small increments in changes of the relative displacement between the pick-up device and the magnetic element. Also, the magnetic pick-up device should exhibit a linear relationship between the output voltage derived therefrom and such displacement, this linearity being present over a relatively wide range of displacement.
Typical semiconductor magnetoresistive devices which have been used heretofore offer less than satisfactory characteristics. Typical of such semiconductor materials, such as gallium-arsenic, indium-antimony, and the like, are highly temperature dependent. Because of such temperature sensitivities, these semiconductor devices exhibit large and diverse variations in the resistivity thereof due to changes in temperature. Hence, when such semiconductor devices are used, a temperature compensation circuit must be coupled thereto in order to compensate for such temperature-dependent diversity in resistivity. Furthermore, in a typical magnetoresistive semiconductor device, the resistivity thereof varies approximately as the square of the magnetic field intensity, provided that the magnetic field is relatively small. These elements generally require a bias magnetic field on the order of about one kilogauss, or more. However, this bias magnetic field generally cannot be applied with sufficient uniformity over a large magnetic region. Thus, magnetoresitive semiconductor devices cannot exhibit the desirable linearity characteristics which are needed for the optimum operation and utilization of magnetic sensors.
Another difficulty in using magnetic sensors to detect the position of a rotary member relates to the magnetic element which must be provided on that member. In general, it is difficult to form the pole face of a metal magnet into an arcuate surface. Hence, bar-shaped magnetic elements typically are used, and these bar-shaped elements are secured to the peripheral surface of the rotary member. However, when such a bar-shaped magnetic element is secured to the rotary member, the center portion of the element generally is tangent to the cylindrical surface of the rotary member, but the opposite ends of the bar-shaped element are disposed at a greater radius from the axis of rotation of the rotary member than is the center portion of the bar-shaped element. Consequently, when the magnetic sensor is fixedly disposed adjacent the rotary path of the bar-shaped magnetic element, the opposite ends of that element rotate in a path that is closer to the device than the path of rotation of the center portion. Thus, the clearance, or spacing, between the magnetic sensor and the bar-shaped magnetic element is not constant. This condition affects the linearity of the output voltage derived from the magnetic sensor and also serves to limit the range over which the linear relationship, if any, obtains.
When using ferromagnetic metal in a magnetic sensor, the resistance thereof, that is, the magnetoresitive effect of the metal, changes as a function of the external magnetic field applied thereto. As explained by Mott's theory, the change in magnetoresistance is a negative change which appears as a linear reduction in the resistivity of the ferromagnetic element as the intensity of the applied magnetic field increases. An isotropic relationship with respect to the direction of the magnetic field obtains when the ferromagnetic material is heated to its Curie temperature. At lower temperatures, however, this isotropic relationship is minimal. In addition, in the presence of relatively small magnetic fields, the resistivity of the ferromagnetic material varies anisotropically with the direction of the applied magnetic field in temperature regions which are below the Curie temperature. For example, for a magnetoresistive element of the type described in the aforementioned patent, wherein a main current path is formed of ferromagnetic material, when the direction of the applied magnetic field is parallel to the path of the current flowing through the material, a maximum resistivity is exhibited. When the applied magnetic field is perpendicular to the direction in which the current flows, this resistivity is a minimum.
This anisotropic relationship is expressed in the Voight-Thomson equation: EQU R(.theta.)=R.sub..perp. .multidot.sin.sup.2 .theta.+R.sub..parallel. .multidot.cos.sup.2 .theta. (1)
wherein .theta. is the angle between the direction in which the current flows and the direction of the applied (preferably saturating) magnetic field, R.sub..perp. represents the resistivity of the material when the current flow direction and the direction of the applied magnetic field are perpendicular to each other, and R.sub..parallel. represents the resistivity of the material when the current flow direction and the direction in which the magnetic field is applied are parallel to each other. This anisotropic characteristic of ferromagnetic materials has been turned to account in various applications, such as those described in U.S. Pat. Nos. 4,021,728, 4,053,829 and 4,079,360. This characteristic also is used in copending application Ser. No. 23,270, filed Mar. 23, 1979 now U.S. Pat. No. 4,296,377.
Some examples of ferromagnetic metals which exhibit desirable magnetoresistive characteristics are nickel-cobalt (NiCo) alloy, nickel-iron (NiFe) alloy, nickel-aluminum (NiAl) alloy, nickel-manganese (Ni-Mn) alloy and nickel-zinc (NiZn) alloy. In magnetic sensors using magnetoresistive elements of the type described in the aforementioned patents, the external magnetic field which is supplied to the sensor typically is generated by a pair of abutting magnets that produce magnetic fields of opposite polarities. As these magnets are disposed relative to the magnetoresistive sensor, the magnetic field applied to that sensor abruptly changes over from one polarity to the other at the boundary line between the magnets. This abrupt change-over, or inversion, in the magnetic field limits the range over which the output signal produced by the magnetoresistive sensor is linearly related to the displacement of that sensor from the boundary line. Consequently, if a desired, or target position of the magnets relative to the sensor is taken as this boundary line, the control system with which the sensor is used should exhibit a desirably high response time.